The development of prefilters to favour the retention of solid matters inside septic tanks goes back about 40 years (U.S. Pat. No. 2,900,084). Such prefilters have spread during the last 20 years, and more particularly during the last 10 years. As an illustration, many American states legally require the installation of prefilters at the outlet of the tanks, in order to limit the charge of particulate matters to be treated at the purifying element following the septic tank.
Already known in the prior art, there are prefilters composed of one or many hollow vertical elements of various forms perforated with openings or slits and enclosed in a housing having an inlet for receiving the waste water and an outlet connected to the outlet of the septic tank in order to allow the evacuation of the filtered water. Examples of such prior art prefilters are given in U.S. Pat. No. 4,439,323; U.S. Pat. No. 5,492,635; U.S. Pat. No. 5,580,453; U.S. Pat. No. 5,382,357 and U.S. Pat. No. 5,482,621. Usually, these prefilters do not provide decantation and/or coalescence phenomenon of the finer particles. The retention is thus assured essentially by the chosen size of the openings whether they be orifices or slits. The feed of these vertical filtering structures can be done from the interior of the element towards the exterior or from the exterior towards the interior, depending on the selected hydraulic flow model. The recourse to such hollow vertical structures favours a partial detachment of the biomass but requires either a filtering element of large dimension, as disclosed in U.S. Pat. No. 4,439,323 or the multiplication of the number of filtering elements, as disclosed in U.S. Pat. No. 5,492,635, and U.S. Pat. No. 5,580,453. If this is not the case, the total available filtration area requires more than one cleaning per year for residences producing a discharge corresponding to an occupation of more than four people.
Also known in the prior art, there are the prefilters of the type disclosed in U.S. Pat. No. 4,710,295 which include a plurality of stacked horizontally extending disc-dam units mounted in vertical in a housing. Each disc is equipped with finger shaped appendices or diverticulums formed by a continuous dam wall that follows a sinuous path and whose top side is open. The superposition and the alignment of the discs by ensuring a spacing between them, helps create a first filtration linear interstice and the sinuous dam-wall creates a second filtration interstice. Therefore, for a given volume, it is possible to increase the available interstitial surface in comparison with the one corresponding to the sum of the set of slits or perforated orifices in a hollow vertical element as described above. In such prefilters, the water to be filtered enters the housing and flows upwardly in vertical channels and then in the horizontal channels formed between each disc where it is filtered. Thus, the control of the particles to be retained is done first at the first filtration interstice. The water then flows horizontally and is further filtered by the dam wall. Usually, the larger particles are retained on the outside of the disc-dam unit and the finer particles have the possibility of decanting in the zone between two horizontal discs leading to the outlet zone.
One drawback encountered with such prefilters is that with time, the accumulation of fine particles and the formation of a biological film between the horizontal discs creates hydraulic restrictions that favour a detachment of the accumulated matters towards the effluent of the septic tank and ultimately requires a cleaning. Cleaning a structure of this type is tedious and may require a complete dismantling of the different elements.
Furthermore, although such prefilters offer a better efficiency than what was known previously, one goal in this field is still to uncover or develop a prefilter being the least voluminous possible and which can offer the maximum filtration capacity.
Therefore, there is still a need at the present time for a filter with a higher efficiency than the existing filters.
Other examples of prior art filter apparatuses for waste water are given in: CA 2,135,937; U.S. Pat. No. 3,332,552; U.S. Pat. No. 5,582,716; U.S. Pat. No. 5,593,584; and U.S. Pat. No. 5,683,577.